Recently, disk recording/reproducing apparatuses have been used for many applications, such as for DVDs, MDs, CDs, CD-ROMs, or the like, which have been diversified year after year. In addition, they have been increasingly improved in density, performance, quality and added value, and reduced in size. Particularly, in disk recording/reproducing apparatuses using recordable magneto-optical media, the demand for those used for data and picture recording tends to increase considerably, and further reductions in size and thickness and further improvement in performance and recording density have been requested.
Conventionally, many techniques relating to optical heads for magneto-optical disks have been reported.
As an example of a conventional disk recording/reproducing apparatus, an optical head for a magneto-optical disk is described with reference to drawings as follows.
FIGS. 10, 11, 12, and 13 are schematic structural views of a conventional optical head and views illustrating the principle of its operation.
In FIGS. 10, 11, 12, and 13, numeral 1 indicates a silicon substrate, numeral 2 a semiconductor laser fixed on the silicon substrate 1, numeral 3 a multisplit photodetector provided on the silicon substrate 1, numeral 4 a resin package, and numeral 5 a hologram element (a diffraction grating) formed of resin, numeral 6 a composite device including a beam splitter 6a, a return mirror 6b, and a polarization separation element 6c, numeral 8 a holder for maintaining an integrated unit 7 including the silicon substrate 1, the semiconductor laser 2, the multisplit photodetector 3, the package 4, the diffraction grating 5, and the composite device 6, numeral 9 a reflecting mirror, numeral 10 an objective lens fixed to an objective lens holder 11, numeral 12 a magneto-optical recording medium having a magneto-optical effect, numeral 13 an objective lens actuator for actuating the objective lens in a focus direction and a radial direction of the magneto-optical recording medium 12, numeral 14 a base as a component of the objective lens actuator 13, numeral 15 screws for tilting, numeral 16 an optical bench, numeral 17 light spots for detecting a focus error signal, which are formed on the multisplit photodetector 3, numeral 18 a light spot for detecting a tracking error signal, which is formed on the multisplit photodetector 3, numeral 19 a main beam (P-polarized light) formed on the multisplit photodetector 3, numeral 20 a main beam (S-polarized light) formed on the multisplit photodetector 3, numeral 21 light receiving areas for a focus error signal, numerals 22 and 23 light receiving areas for a tracking error signal, numeral 24 light receiving areas for an information signal, numeral 25 subtractors, numeral 26 an adder, numerals 27 and 28 focal points of the light spots for detecting a focus error signal.
With respect to the conventional example with the configuration as described above, its operation is described as follows.
A beam emitted from the semiconductor laser 2 is separated into a plurality of different beams of light by the hologram element 5. The plurality of different beams of light pass through the beam splitter 6a in the composite device 6, are reflected by the reflecting mirror 9, and then are focused on the magneto-optical recording medium 12 as a light spot 30 with a diameter of about 1 μm by the objective lens 10 fixed to the objective lens holder 11.
A beam of light reflected by the beam splitter 6a in the composite device 6 enters receiving optics for monitoring the laser (not shown in the figures) to control a driving current for the semiconductor laser 2.
Reflected light from the magneto-optical recording medium 12 travels along the reverse path to be reflected and separated by the beam splitter 6a in the composite device 6, which is incident on the return mirror 6b and then the polarization separation element 6c. 
The semiconductor laser 2 is mounted so that the polarization direction is parallel to the surface of the paper in FIG. 11A. Incident light is separated into two beams of light whose polarized components are orthogonal to each other by the polarization separation element 6c, which are then incident on the light receiving areas 24 for an information signal.
Out of the reflected light from the information recording medium 12, a beam of light that has passed through the beam splitter 6a is separated into a plurality of beams of light by the diffraction grating 5, which are focused on the light receiving areas 21 for a focus error signal and the light receiving areas 22 and 23 for a tracking error signal.
Focus servo is operated by a so-called SSD (spot size detection) method and tracking servo by a so-called push-pull method.
Further, by calculating the difference between the main beam 19 composed of P-polarized light and the main beam 20 composed of S-polarized light, a magneto-optical disk information signal can be detected by a differential detection method. Furthermore, by calculating the sum of them, a prepit signal can be detected.
The reflecting mirror 9 is fixed to the optical bench 16. The integrated unit 7 is bonded and fixed to the holder 8, which is then fixed to a side face of the optical bench 16.
In the optical head with the configuration as described above, in order to obtain a desired detection signal through reflected light from the magneto-optical recording medium, the relative position adjustment of the semiconductor laser 2, the objective lens 10, and the multisplit photodetector 3 is carried out at the time of their assembly.
With respect to such relative position adjustment, in the above-mentioned conventional apparatus, the position of the multisplit photodetector 3 in a Z′-axis direction (an optical axis direction, see FIG. 10) is determined uniquely by specifying the sizes of the optical bench 16 and the holder 8 so that the light receiving plane is positioned approximately midway between the focal points 27 and 28 of light spots. A focus error signal and a tracking error signal are adjusted by maintaining the holder 8 with an external jig and moving the integrated unit 7 in an X direction and a Y direction (see FIG. 10) so that approximately equal outputs are obtained from the light receiving areas 22 and 23 for a tracking error signal.
On the other hand, the relative tilt between the magneto-optical recording medium 12 and the objective lens 10 is adjusted by so-called “spherical tilting” that is carried out by rotating the screws 15 for tilting to bring a spherical slide portion of the base 14 as a component of the objective lens actuator 13 and a spherical slide portion of the optical bench 16 into contact and then to allow the base 14 to pivot. In this case, the center of rotation is the principal point 10a of the objective lens 10. In FIGS. 10 and 13, θR indicates skew adjustment in a radial direction (the adjustment by the rotation about the Z′-axis) and θT indicates skew adjustment in a tangential direction (the adjustment by the rotation about the X-axis).
However, in the above-mentioned conventional configuration, the relative position adjustment of the semiconductor laser 2, the objective lens 10, and the multisplit photodetector 3 is carried out by moving the holder 8 maintaining the integrated unit 7 in the X-axis direction and the Y-axis direction. Therefore, it is necessary preliminarily to provide a margin for the adjustment. Particularly, the margin for the adjustment in the Y-axis direction leads directly to the increase in overall height (the height in the thickness direction of a magneto-optical recording medium) of the optical head.
Further, since the integrated unit 7 is minute, it is difficult to adjust its position individually. Therefore, it is necessary to adjust its position through the holder 8 maintaining the integrated unit 7. Consequently, the wall thickness of the holder 8 in the Y-axis direction also contributes to the increase in overall height of the optical head.
Thus, as long as the relative position adjustment of the semiconductor laser 2, the objective lens 10, and the multisplit photodetector 3 is carried out in the conventional configuration, the reduction in size and thickness of the optical head and the disk recording/reproducing apparatus is difficult, which has been a problem.